Targeting frustration: a review on reducing abnormal behaviors in broiler breeders via environmental enrichment

1 Introduction

The demand for chicken meat has been steadily growing over the past decades (USDA, 2024). To meet this demand, broilers have undergone genetic selection to optimizing growth rate and body mass (Decuypere et al., 2010). This has caused an increased weight gain in broilers over the years, going from 905 grams in 1957 to 1,808 grams in 1978 and 4,202 grams in 2005 by 56 days of age (Zuidhof et al., 2014). Currently, two of the most common high-yielding broiler lines reach around 4,710 grams for Aviagen Ross 308 (Aviagen, 2022), and 4,950 grams for Cobb 500 (Cobb, 2022) at day 56.

While broilers are selected primarily for rapid growth and weight gain efficiency, broiler breeders must not only possess these same growth-related traits but also maintain sufficient reproductive performance, through traits like good egg production, fertility, hatchability, and sperm quality (Sweeney et al., 2022). Unfortunately, high body weight and reproductive efficiency have been proven to be negatively correlated, resulting in reduced reproductive performance if breeders are fed ad libitum (Decuypere et al., 2010; De Jong and Guémené, 2011). Therefore, a balance needs to be found that allows the genetic potential for growth desired for broilers and maintaining high reproductive efficiency. For this purpose, management strategies like feed restriction have been implemented to prevent reproductive issues and ensure birds are at the desired weight.

Feed restriction consists of limiting the quantity or quality of food provided to broiler breeders. Broiler breeders undergo the most severe feed restriction during the rearing phase, when birds are often placed on a skip-a-day schedule during which they are fed every other day (Aviagen, 2024). In the rearing phase birds receive about 25% of the feed that birds ordinarily eat when fed ad libitum (Carney et al., 2022). Although feed restriction is effective at ensuring birds maintain reproductive weight, it results in a paradox where feed restriction prevents excessive weight gain, ensures optimal reproductive performance, and prevents obesity but simultaneously creates welfare issues for the birds (Decuypere et al., 2010). Evidence shows that broiler breeders subjected to feed restriction are chronically hungry (Savory and Maros, 1993; Mench, 2002), and have unfulfilled behavioral needs (De Jong and Guémené, 2011), resulting in frustration, stress, boredom, and aggression. One of the ways this is manifested is through the performance of abnormal behaviors, such as repetitive pecking, feather pecking, pacing, and cannibalism (Wood-Gush, 1958; Rodenburg et al., 2005; De Jong and Guémené, 2011). The appearance of abnormal behaviors can be indicative of reduced welfare, and contemporary studies highlight how environment and management practices can influence birds’ responses and the performance of abnormal behaviors (Riber et al., 2017).

The welfare of farm animals is becoming an increasingly more important consideration in modern agricultural (Mellor and Webster, 2014). As society is starting to recognize the sentience of farm animals, and thus their ability to experience pain and discomfort, there is an increase in expectations to decrease suffering and maximize positive welfare states. This is a global trend, as illustrated in legislative reforms, certification programs, and consumer-driven demand for higher welfare standards seen throughout the European Union, North America, and beyond. Improving animal welfare is not only an ethical imperative, but also a practical one. Practices that enhance welfare are often cost-neutral or economically advantageous. As animal health, affective state, and productivity are closely linked (Appleby et al., 2022), management practice that ensure good welfare, allow for improved productivity and reduced losses, due reduced injuries, stress, mortality, improved health and lower disease risk (Dawkins, 2017). Therefore, welfare-friendly practices can be beneficial for the birds and for sustainable production.

Environmental enrichment has been suggested as an effective strategy to mitigate abnormal behaviors and improve welfare (Riber et al., 2017). Enrichment can help channel frustration, provide opportunities to perform behavioral needs, and enhance overall well-being by providing opportunities for interaction with the environment (Newberry, 1995).

The scarcity of research available on enrichment usage on broiler breeders highlights the need for more scientific knowledge on environmental enrichment and best practice to help improve bird welfare. This review aims to explain the abnormal behaviors present on broiler breeders as well as comprehensively examine the current literature on potential environmental enrichment options for broiler breeders and highlight existing research gaps.

2 Methods

To identify relevant scientific literature, a comprehensive search was conducted across several academic databases, including the Auburn University library (AU library), Web of Science, ScienceDirect, and Cambridge University Press. A combination of keywords related to broiler breeder environmental enrichment, frustration, and welfare were employed. These keywords included “broiler breeder environmental enrichment,” “enrichments for poultry,” “frustration in broiler breeders,” “enrichments for broiler breeders,” and “broiler breeder welfare.” Additionally, reference lists of identified articles were scanned to capture any relevant articles and conference abstracts not identified through the initial keyword search. Only peer-reviewed articles were included. Studies published on or after 2000 were included to ensure the research; as well as two older articles were incorporated due to their relevance. Older literature is also cited throughout the article for specific concepts and terminology.

3 Abnormal behaviors in broiler breeders

Abnormal behaviors are those that lack purpose and are often invariant, atypical, and out of context (Garner, 2005), these behaviors may also be considered as undesirable behaviors or behavioral disorders (Hockenhall and Creighton, 2010; Haq et al., 2022). The presence of abnormal behaviors is usually an indicator of reduced welfare as they often arise when animals are unable to experience stimuli that allows them to express their normal behaviors (Rushen and Mason, 2006; Broom, 2019; Rotta et al., 2023). In some cases, these behaviors develop as adaptative responses to overcome restrictions (Lewis and Bodfish, 1998; Langen et al., 2011; Appleby et al., 2022). The environment also plays a big role in the development of abnormal behaviors (Weidenmayer, 1997). Environments that prevent animals from performing natural behaviors can lead to difficulties in adapting to their environment and can result in development of abnormal behavior, with the abnormal behaviors helping animals cope with adverse environments (Mason, 1991, 2006; Garner, 2005; Bracke and Hopster, 2006; Broom, 2019).

In broiler breeders, management factors, such as feed restriction and an environment with a lack of complexity, play a huge role in the appearance of abnormal behaviors that are indicative of reduced welfare (Estevez, 2009). This is particularly relevant during the rearing phase, when feed restriction is more severe due to skip-a-day programs, resulting in greater periods of feed deprivation compared to the production phase.

One of the abnormal behaviors that has been identified in broiler breeders is pacing (Hocking et al., 1997). Pacing is defined as walking back and forth without any specific purpose and is often attributed to limited space and lack of complexity in the environment (Rose et al., 2017). This problem is commonly identified in isolated laying hens and breeders, highlighting the importance of the environment’s role in the appearance of pacing.

Polydipsia, or excessive drinking, arises when the breeder is unable to fulfil feeding motivation and replaces it with drinking ( (Mench, 2002; van Emous, 2024). This is an abnormal behavior that is commonly found in broiler breeders (Savory et al., 1991; Carvalho et al., 2022). With feed restricted birds consuming approximately 2.2 times the amount of water to non-feed restricted birds (Hocking, 1993), this can cause some welfare concerns as repetitive pecking to the drinkers can cause water spillage, which results in wet litter and increases the risk of contact dermatitis (van Emous, 2024; Weaver et al., 2025), such as foot pad dermatitis, hock burn, and breast blisters. As a result, 65% of breeder farmers have limited water access to prevent polydipsia (van Emous, 2024), which creates additional potential welfare issues.

Another abnormal behavior that has been identified in broiler breeders is repetitive pecking at inanimate objects. This is when a bird pecks repeatedly, in a non-exploratory way, at objects such as litter, drinkers, empty feeders, and pen walls (Dixon et al., 2008; De Jong and Guémené, 2011). Repetitive pecking has been attributed to the lack of satiety caused by the feed restriction, and the birds redirecting their feed seeking motivation to other objects (Bessei, 2015).

An additional abnormal behavior documented in broiler breeders is pecking at conspecifics. This behavior can take several different forms, namely, i) Feather licking/sucking, defined as a gentle pecking at the edges of or mouthing of the feathers of a conspecific, usually unnoticed by the recipient (Leeson and Walsh, 2004; Taylor et al., 2024); and ii) Feather pecking, defined as pecking at and removal of the feathers of a conspecific (Savory, 1995; Dixon et al., 2008). These abnormal behaviors are a cause of concern as they can spread throughout a flock, increasing the damage done by these behaviors. Furthermore, the behaviors can cause significant feather loss in the recipients (Taylor et al., 2024), which can lead to injuries, discomfort, and thermoregulation issues (Guhl, 1958; van Staaveren et al., 2021). Given challenges present with broiler breeders’ welfare, interventions should prioritize managing the occurrence of abnormal behaviors such as repetitive pecking, feather pecking, feather licking, pacing, and polydipsia, by providing suitable objects to that enrich the environment and allow birds to express their normal behaviors and reduce their frustration (Piepho et al., 2017).

4 Environmental enrichment

Environmental enrichment aims to enhance the complexity of an animal’s environment (Newberry, 1995). This can be achieved through modifications of existing structures or the introduction of novel objects within the animals’ housing. Alongside improving environmental complexity, enrichment also plays a crucial role in helping animals channel frustration and express natural behaviors, through improving sensory stimulation, targeting intrinsically motivated behaviors (internally rewarding), and providing the animals more control over their life and environment (Bloomsmith et al., 1991; Newberry, 1995; Young, 2003; Tarou and Bashaw, 2007; Brandes et al., 2022).

To select ideal enrichments for broiler breeders, it is important to identify which behavioral need is not being met. When birds lack objects such as food or forage items to satisfy their pecking motivation, they seek similar sensory impressions like pecking (Hoffmeyer, 1969). An example of this which often occurs during the rearing period is repetitive pecking, which is driven by feed restriction (Mench, 2002), and can escalate to feather licking, feather pecking, and cannibalism (Savory, 1995). To target abnormal behaviors and improve broiler breeder welfare a potential option is the addition of enrichments that will satisfy sensory needs and can reduce stress by channelling frustration.

The research on environmental enrichment and broiler breeders has investigated the use of perches, bales of substrate, pecking blocks, cover panels, and hanging pecking objects. This research will be reviewed below, but due to the scarcity of research on environmental enrichment and broiler breeders, the information has been supplemented with research on other poultry species when appropriate.

4.1 Perches

One of the most widely researched enrichments in broiler breeders is perches. Perches are elevated structures, on which birds can perform perching and roosting behavior (Brake, 1987). Research has found that breeders exhibit a preference for high resting places due to an instinctual motivation to protect themselves from predators, with evidence that breeders start perching at 2 weeks of age (de Jong and van Emous, 2017). With perching being a natural behavior, the expression of perching and roosting from an early age is encouraged (Gebhardt-Henrich et al., 2017; Bist et al., 2023).

Due to the unique body conformation and size of the broiler breeders, perch design needs to be carefully considered (Malchow et al., 2019; Brandes et al., 2022). Some research has looked into different aspects of perch design and management such as the optimal material, height, and age (Table 1). In broiler breeders, several materials have been studied, including wood, steel, plastic, metal, and rubber-coated perches (Gebhardt-Henrich et al., 2017; Brandes et al., 2020; Vasdal et al., 2022; Vasdal et al., 2022; Baxter and O’Connell, 2025). Vasdal et al. (2022) studied bird preference for perch material, shape, and height by comparing steel round, steel square, plastic mushroom shaped, and round wooden perches. They found that breeders exhibited both a preference for mushroom shaped plastic perches and the highest perch available when analysing the number of birds using the perches (Vasdal et al., 2022). Preference for high perches was also observed by Brandes et al. (2020), who evaluated the preference in between perches at 38 to 48 cm over the litter and perches 0 to 15 cm over the slated area and found that breeders exhibit a preference for perches that were at least 5.5 cm high.

Table 1. Summary of available literature on perches usage with broiler breeders.

Research has also shown that bird age may impact perch usage, with different studies finding conflicting trends. Namely, one study reports a decline in perching behavior with increasing age (Gebhardt-Henrich et al., 2017), while another study reported that perch behavior was constant with age (Vasdal et al., 2022). More research is needed to understand the relationship between age and enrichment usage. One important consideration for research measuring the use of perches is that previous studies have reported that the use of perches increases at night (Gebhardt-Henrich et al., 2017; Baxter and O’Connell, 2025), meaning this is an important time for evaluating perch use and preference.

Currently, there is limited research focus on the reduction of abnormal behaviors using perches in broiler breeders. However, there is evidence in laying hens that the provision of perches reduced the occurrence of feather pecking (Huber-Eicher and Audigé, 1999). Moreover, perches have shown to reduce fearfulness in broiler breeders, as shown by shorter scores in tonic immobility (Brake et al., 1994). This was a result of the breeders being able to use the perches to escape from injurious pecking from dominant hens (Brake, 1987; Brake et al., 1994). Furthermore, by avoiding aggressive interactions, breeders also improved feather condition and reduce overall stress (Bist et al., 2023).

An additional benefit of perches is that they reduce the amount of contact birds make with wet litter, reducing the incidence and severity of footpad dermatitis (Vasdal et al., 2022). Research has also found that perches have a positive effect on survival under heat stress conditions, due to birds being able to thermoregulate better on the perches rather than the floor (Brake, 1987). Alongside these welfare benefits, perches have also shown a benefit to production by reducing mortality as well as the incidence of eggs being laid on the floor rather than in the designated nest (Brake, 1987; Gebhardt-Henrich et al., 2017). Nevertheless, perches have also been associated with keel bone damage and breast blisters (Gebhardt-Henrich et al., 2018; Mens and van Emous, 2022). The appearance of keel bone damage caused by perches on broiler breeders needs to be further studied, since it negatively affects welfare by being painful, and has been linked to influence reproductive performance on laying hens (Riber et al., 2018). Overall, perches seem like a great potential enrichment for broiler; however, more research is needed to understand their impact on keel bone damage and abnormal behaviors in broiler breeders.

4.2 Bales of substrate

Bales of substrate are compacted foraging materials that come in various presentations, materials, and sizes. Some of the bale materials that have been researched as enrichment use in poultry are straw, wood shavings, and hay (Edmond et al., 2005; Hocking and Jones, 2006; Baxter et al., 2018; Mocz et al., 2022). The presentation of the bale also differs between studies, with some studies keeping the substrate uncovered (Mocz et al., 2022) and other studies keeping the plastic wrap, as this could be more attractive for the birds and encourage engagement (Hocking and Jones, 2006; Baxter et al., 2018; Quino et al., 2024) (Table 2).

Table 2. Summary of available literature on bales of substrate usage with broiler breeders and broilers.

The provision of bales of substrate has been shown to provide opportunities for the performance of foraging behavior, thereby increasing these behaviors (Hocking and Jones, 2006; Baxter et al., 2018), with research showing that when provided with enrichment during rearing, birds continue to engage with and use the enrichment even months after initial placement (Quino et al., 2024). Increased foraging associated with providing birds with bales may also reduce the incidence of repetitive pecking and feather pecking, as these behaviors have been associated with the lack of foraging opportunities (McAuley et al., 2025).

Bales of substrate provide many different opportunities for interaction (Baxter et al., 2018), alongside the aforementioned foraging, it also allows for behaviors such as dustbathing, resting, perching, and explorative pecking (Ohara et al., 2015). Furthermore, research with bales of substrate in broilers has reported elevated activity levels leading to improved leg health, walking ability and reduced incidence of hock burns (Bailie et al., 2013; Baxter et al., 2018; Mocz et al., 2022). Bales of substate have been shown to reduce stress, as measured by Heterophil: Lymphocyte ratios (Ohara et al., 2015). It has been suggested that because bales of substrate provide birds opportunities for the performance of normal behaviors, they alleviate stress and frustration, particularly frustration related to behaviors that arise from hunger related to feed restriction (Edmond et al., 2005). Frustration is a negative emotional state that arises from unfulfilled behavioral needs, measured by the performance of abnormal behaviors (Jones et al., 2020).

In regard to production, bales of substrate have been linked to improved eggshell quality (Edmond et al., 2005) with no negative effects on production being reported. The improvement in eggshell quality was attributed to early interaction with enrichment, reducing stress and aggressive interactions, therefore increasing the hen’s ability to produce higher-quality eggshells (Edmond et al., 2005). Nevertheless, it is also important to highlight that Hocking and Jones (2006) did not find a significant effect of bales of substrate on reducing frustrated feeding behaviors when compared to substrate used as bedding. Another consideration is the appropriate selections of material, as certain materials like straw have been reported to increase moisture-induced dermatitis (Jones et al., 2020).

Although there is limited research on the use of substrate bales on broiler breeders, they seem like a good potential option as they are considered available and affordable (Edmond et al., 2005). However, further research is needed to fully understand the impacts and the best methods for using bales of substrate.

4.3 Pecking blocks and pecking stones

Pecking blocks and stones are edible enrichment material that can be used in raising poultry (McAuley et al., 2025; Ehigbor et al., 2025a). There are several different types available and can be made of various different types of materials such as grain based corn, wheat, oats, and minerals (ViloFoss, 2020; McAuley et al., 2025) (Table 3). Pecking blocks are also available in different hardnesses and shapes, with manufacturers recommending different types of pecking stones depending on the poultry (ViloFoss, 2020). Pecking blocks are normally introduced to poultry to encourage pecking and foraging behavior (Kwon et al., 2024; McAuley et al., 2025), and thereby redirecting and reducing harmful behaviors, such as feather pecking, aggression, and cannibalism (ViloFoss, 2020). Allopecking has been associated with misdirected foraging and explorative behavior (Schreiter et al., 2020a; McAuley et al., 2025).

Table 3. Summary of available studies on pecking blocks usage with laying hens and broiler breeders.

Some of the aspects of pecking stone usage that have been studied in laying hens are how the hardness of pecking stone, beak trimming, and presence of keel bone fractures impact usage (Table 3). In regard to hardness, research focusing on laying hens during the rearing period found that laying hens showed a preference for the medium pecking stones over the hard pecking stones (McAuley et al., 2025). Another study relating birds’ usage of pecking stones to keel bone fractures, found that birds are more likely to use pecking blocks if they had an existing fracture (Ehigbor et al., 2025a). This is likely reflecting their calcium demand (Ehigbor et al., 2025a). Another study reported a greater use of the pecking stones if the hen is beak trimmed (Iqbal et al., 2020). It has also been reported that in laying hens pecking block usage increased with time (Iqbal et al., 2020; Ehigbor et al., 2025a). Broiler breeders were also found to have both an initial interest and sustained usage of pecking stones when used as an enrichment (Quino et al., 2024).

Pecking blocks have been proven effective on targeting abnormal behaviors, reducing the incidence of feather pecking and repetitive pecking in laying hens (Iqbal et al., 2020; Baker et al., 2022). This in turn reduced injurious pecking (Baker et al., 2022), skin injuries (Schreiter et al., 2020b), keel bone fractures (Ehigbor et al., 2025a), cannibalism (Schreiter et al., 2020b), and mortality (Iqbal et al., 2020). Furthermore, pecking blocks also promoted foraging and dustbathing behaviors (Ehigbor et al., 2025b) and improved feather coverage scores and feather damage (Iqbal et al., 2020; Schreiter et al., 2020b). A natural reduction in beak length was also found to occur in laying hen pullets, leading to a decrease injurious pecking in pullets (Baker et al., 2022). Additional advantages of pecking blocks include, improve eggshell breaking strength and thickness, with no reported negative impact on egg production in laying hens (McAuley et al., 2025; Ehigbor et al., 2025a). In broilers, similar results were found with pecking stones not altering productivity or welfare indicators (Kwon et al., 2024).

It should be noted that although there are various studies looking at pecking stones with laying hens, there is limited research published on the effect of pecking blocks on broiler breeders. The existing research with broiler breeders does show preference for pecking stones over shaving bags (Quino et al., 2024), and that pecking stones cause a reduction of oral stereotypies (Evans et al., 2023). This is promising and suggests that pecking blocks could be a potential enrichment for broiler breeders, but further research is needed to fully understand their impacts.

4.4 Cover panels

Cover panels are mesh screen panels attached to support stand placed in the litter area of the floor. The birds are able to use the panels to perch, but can also rest against or on top of them. The panels are elevated and thus encourage perching, while the mesh allows birds to rest against them and use them as a refuge. They also encourage a more even distribution of the hens throughout the house (Leone and Estévez, 2008; Estevez, 2009). In addition, cover panels also reduce injuries to the head and wings (Leone and Estévez, 2008) and improve rest by reducing disturbances (Cornetto and Estevez, 2001). Cover panels also benefit reproductive outcomes, as they improved egg production, hatchability, fertility, and increased the laying period (Leone and Estévez, 2008). Even though there is very limited research on the use of cover panels in broiler breeders, the results have been promising, and further research is needed into this enrichment option.

4.5 Hanging pecking enrichments

In the literature on broiler breeders, two hanging pecking enrichments have been studied. These are hanging reflective spheres (Baxter and O’Connell, 2025) and hanging strings (Hocking and Jones, 2006; Evans et al., 2023). These hanging objects aim to target the instinctive pecking behavior, thereby reducing stress (Kwon et al., 2024). The studies performed on hanging enrichments for broiler breeders studied their effects in combination with other enrichments such as substrate, perches, and pecking blocks, making it hard to comment on their individual effect of the hanging objects specifically. However, the research did find that both the reflective spheres and hanging strings attracted broiler breeders and helped redirect the repetitive pecking to explorative pecking at the objects (Hocking and Jones, 2006; Evans et al., 2023; Baxter and O’Connell, 2025). The hanging objects, combined with other enrichments, did not seem to result in any negative impacts on egg production, mortality, feather score, and floor eggs (Baxter and O’Connell, 2025). As there is no research on the effects of hanging pecking objects by themselves, it is hard to comment on their use as sole enrichment. However, available research suggest that they can be used as a complementary enrichment to encourage pecking, as no negative effects on reproductive performance having been found.

5 Special consideration for enrichment usage

When identifying the correct enrichment, it is important to consider that the structure of the enrichment will not cause a problem for the broiler breeder. Enrichment is a useful tool for improving the welfare of animals; however, it is important to ensure that enrichments are used properly and are correctly designed to avoid welfare, management, and production complications, as some studies reporting hanging enrichment can cause entanglement (Hocking and Jones, 2006; Estevez, 2009), wooden perches can cause breast blisters (de Jong and van Emous, 2017), straw used to forage can increase moisture-induced dermatitis (Jones et al., 2020), and shaded structures like bales can result in high incidence of floor eggs (Lopez et al., 2022).

Another important consideration, regardless of the type of enrichment, is the number of enrichments used in relation to the number of animals. It is important to ensure there is sufficient availability of the enrichment to meet everyone’s needs; otherwise, the enrichment could create competition resulting in a reduction of welfare (Barnard et al., 1996; Estevez, 2009). Additional research is also needed to elucidate the best practices for enrichment implementation and design, to ensure that enrichment is a tool that can help improve breeders’ welfare without creating management and production complications.

6 Research gaps

Currently, there are still many of existing research gaps regarding enrichment usage in broiler breeders. Although research has shown a few environmental enrichment options with great potential for broiler breeders, further research is needed to understand the full effects of these enrichment types on the welfare and reproductive success of the birds, especially how the benefits of enrichment are impacted by growth and production stage. For example, during rearing, the use of feed restriction and skip-a-day feeding can lead to the performance of more abnormal behaviors, providing occupational enrichments can redirect these behaviors, especially on the days that breeders are not being fed. Whilst during the egg production period, enrichments could improve hens’ welfare by allowing them to escape unwanted or painful mating interactions, leading to more beneficial impact on hens’ welfare plus improving production (Hocking and Jones, 2006).

Further research is needed on the impact of environmental enrichment for broiler breeders and its effects on welfare outcomes across different strains, densities, and ages, as these are significant factors for behavioral expression (Whittle et al., 2025). Since most studies use other poultry as a baseline for enrichment, establishing a broiler breeder specific baseline would help better target specific abnormal behaviors and improve welfare. In addition, economic studies comparing enriched with non-enriched flocks are needed to assess the effects of environmental enrichment on reducing losses caused by mortality or injuries by the comparison enriched and non-enriched flocks.

7 Conclusion

The implementation of environmental enrichment in broiler breeder management represents a promising approach to improving bird welfare and mitigating abnormal behaviors associated with feed restriction and confinement. Research has found that enrichments such as pecking blocks, bales of substrate, perches, cover panels, and pecking objects provide opportunities for exploration, thereby encouraging natural behaviors and reducing frustration. However, enrichment strategies must be carefully designed to avoid physical risks, compromise welfare, and ensure they do not negatively impact production parameters. Future research should focus on discovering new enrichment options, applicability, density, and placement to balance welfare benefits with practical and economic considerations, contributing to more sustainable and ethical broiler breeder production systems.

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Footnotes

Edited by: Domenico Vecchio, Experimental Zooprophylactic Institute of Southern Italy (IZSM), Italy

Reviewed by: Cristiano Azevedo, Universidade Federal de Ouro Preto, Brazil

Scott Nunes, University of San Francisco, United States